When mechanical springs are used in a railway car, there is generally no levelling or adjustment means included to maintain the height of the car body a predetermined distance from the ground. This is not true when air springs are used to support the car body, in which case levelling valves are operated to control the air in the springs to maintain the car body level at a predetermined distance from the ground.
When trucks are operated over long periods of time, the wheels of the wheel axle units connected thereto tend to wear. As the wheels wear, the height of the car body tends to lower with respect to ground level. Heretofore, when mechanical springs were used, there has generally been no convenient apparatus for adjusting the level of the car body with respect to the track level.
In the past, in order to adjust for variations in the height of the car body to compensate for wheel wear, it was necessary to first disassemble the truck from the car body. Shims then had to be inserted below the springs. The number of shims which had to be added depended upon the amount of wheel wear.
Periodically disassembling a truck from a car body to permit insertion of shims is time consuming and very costly. Loose shims are also inconvenient to use and generally are disgarded after use.
In many cases, shock absorbers are used inside of the mechanical springs. It is desirable to be able to replace these shock absorbers without disturbing the assembly of the car body and truck including the spring arrangement.